A recent solid-state image pickup device utilizing a semiconductor has its chip area reduced by shrinking the size of a pixel device. As a result, the cost per chip can also be lowered as well. In addition, the size of an image pickup apparatus (or a camera system) itself can also be decreased.
The image pickup device includes mainly a photoelectric conversion device for converting a light beam coming from a lens into a signal electric charge and a transfer CCD (Charge Couple Device) for delivering the signal electric charge to an amplifier for converting the signal electric charge into an output voltage. In a typical configuration of a pixel cell of an image pickup device, a photoelectric conversion device and a transfer CCD are laid out horizontally. With such a layout, however, it becomes practically difficult to shrink the size of the image pickup device during a fabrication process. In order to solve this problem, a solid-state image pickup device has been proposed recently as a device with a layered structure in which a photoelectric conversion device is provided above a transfer CCD. For more information, refer to documents such as pages 3 and 4 as well as FIG. 2 of Japanese Patent Laid-open No. 2001-257337.
In accordance with this conventional technology, however, transfer CCDs provided below a photoelectric conversion device for RGB (Red, Green and Blue) pixels are not separated away from each other. For this reason, a light beam having a large wavelength as is the case with a red-color light beam for a red-color pixel of the RGB pixels penetrates the photoelectric conversion device and hits the transfer CCDs to raise a smear problem caused by mixing of a noise signal with signal electric charge being transferred. In order to solve this problem, a generation rate of smears for the depth of the semiconductor area of the transfer CCD was examined with the depth of a photosensor taken as a parameter. Results of the examination are shown in FIGS. 9A and 9B. In FIGS. 9A and 9B, the depth of a photosensor is taken as a parameter, the vertical axes each represent the generation rate (or the cumulative generation rate) of smears and the horizontal axes each represent the depth of the semiconductor area of the transfer CCD. To be more specific, FIG. 9A shows examination results for a red-color light beam having a wavelength of 700 nm. On the other hand, FIG. 9B shows examination results for a green-color light beam having a wavelength of 550 nm. In the case of a structure with X=5 μm and ΔX=0.5 μm where notation X denotes the depth of the photosensor whereas notation ΔX denotes the depth of the semiconductor area of the transfer CCD, for example, the generation rate of smears for a light beam with a wavelength of 700 nm (that is, for a red-color light beam) approaches about 4% as shown in FIG. 9A. For a light beam having a wavelength of 550 nm (that is, for a green-color light beam), on the other hand, the generation rate of smears is about 1% as shown in FIG. 9B. As is obvious from the results of the examination, the generation rate of smears for a red-color light beam is extremely high.